Why Extraction Chemistry Matters
Soil testing does not measure total elemental reserves; it measures the plant-available nutrient pool using chemical solutions designed to mimic root acid exudates. Choosing a laboratory that uses the proper extraction method calibrated to your geographic soil type is critical for accurate lime and fertilizer recommendations.
1. Mehlich-3 Extraction (Universal Multielement Extractant)
Optimal Soil pH: 5.5 to 7.4 (Acid to slightly alkaline soils)
Geographic Scope: Widely adopted across the Midwest, Mid-Atlantic, Southeast, and Southern Plains.
Advantages: Extracts Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sodium (Na), and micronutrients (Zn, Mn, Cu, B) in a single ICP run, providing accurate cation exchange capacity (CEC) and base saturation figures.
2. Bray-1 Phosphorus Extraction (Bray P-1)
Optimal Soil pH: Below 7.2 (Acidic to neutral soils)
Geographic Scope: Traditional benchmark standard in the Corn Belt (Illinois, Iowa, Indiana, Ohio, Minnesota).
Limitations: In calcareous soils (pH > 7.3), free calcium carbonate neutralizes the acid in the Bray solution, causing severe underestimation of available phosphorus.
3. Olsen Sodium Bicarbonate Extraction
Optimal Soil pH: 7.3 to 8.5+ (Alkaline, calcareous, and high-free-lime soils)
Geographic Scope: Standard calibration in the Western United States, Pacific Northwest, California Central Valley, and Northern Great Plains.
Mechanism: Uses 0.5 M NaHCO3 at pH 8.5 to precipitate excess calcium ions, allowing accurate measurement of phosphorus in high-pH soils.
4. Morgan & Modified Morgan Extraction
Optimal Soil pH: Northeast acidic soils and organic sandy loams
Geographic Scope: Standard calibration across New York, New England, and parts of Canada (Cornell & UVM soil calibrations).